This course is part of the Custom Home Academy

Factory vs. Field

Architects should understand how and why to consider and specify for factory versus field mulled window assemblies. Factory mulled windows are often considered ideal because the factory approach usually ensures that each window unit is joined together with precision, resulting in consistent sizing, alignment, and performance. This is especially important when creating larger window units, where any inconsistencies could result in air leaks, water infiltration, or other issues. There is also generally greater quality control since the manufacturer can check that each window unit meets its specifications before it is shipped. Choosing factory mulling can also provide greater warranty protection since the manufacturer is responsible for the entire window unit, including the mullion joint. And that warranty can provide peace of mind for both the homeowner and the contractor. In addition, factory mulling can save time and labor costs since the window units are already pre-assembled and ready to install.

Field mulling often faces challenges with weather conditions, materials, and lack of assembly stations. That being said, field mulling can be a viable option in certain situations such as when working with unique or custom window configurations that are harder to achieve through factory mulling. It may also be preferred and necessary with some big-glass designs in situations where projects must ship in sub-assemblies and transportation or accessibility to the job site is a concern. Some companies now offer options for quick on-site assembly by delivering large factory-mulled sub-assemblies, with all necessary components and instructions included, so that contractors have a much easier, faster assembly prep kit that results in an AAMA-450-certified window assembly. This type of assembly solution can help speed construction timelines by incorporating all of the necessary fasteners, clips, caps, and other parts—which reduces specifying time and the risk of missing parts and lost time at the jobsite.

AAMA 450

American Architectural Manufacturers Association (AAMA) 450 is the certification standard for the testing and rating of windows, doors, and skylights. It is useful to both engineers and manufacturers and makes compliance of mulled systems easier and more effective. Specifically, it is a performance standard for the thermal transmittance (U-factor) of fenestration products— how well a window, door, or skylight prevents heat from escaping a building. The lower the U-factor, the better the insulation value of the fenestration product. AAMA 450 sets performance requirements for U-factors for various types of fenestration products under different test conditions.

Mulled assemblies that are not built or tested to meet this standard may not meet performance expectations. Especially in extreme weather conditions and in particular locales, such as coastal regions, underperformance can be a hazard to the safety of building occupants.

Originally published in 2000 as AAMA 450-00, Voluntary Performance Rating Method for Mulled Fenestration Assemblies, the document defined procedures for arriving at a meaningful design pressure (DP) rating from which air, water, and structural performance of mulled assemblies could be more effectively assessed. The standard has been updated since, including in response to major events like the 2004 and 2005 hurricane seasons, when the U.S. experienced a series of severe storms. Hurricanes Charley, Frances, Ivan, and Jeanne caused a combined $62.2 billion in damage and resulted in 123 deaths in 2004. In 2005, a total of 28 named storms hit the U.S., including most notably Hurricane Katrina, which devastated the city of New Orleans, caused more than $125 billion in damage, and resulted in over 1,800 deaths.

AAMA 450 offers three options for determining the overall DP rating of a mulled fenestration system, which is the lower design pressure achieved by either an individual product or by the mulled system, as determined by testing, calculation, or a combination of the two.

The first option tests the total system configuration, which must meet the minimum performance levels for air leakage, water resistance, and uniform load as outlined in the North American Fenestration Standard (NAFS). The second option tests the mullion elements of individual components using a simple beam concentrated load test with the load applied at the center span. Deflection under the load and permanent deflection after the load removal are recorded and must be less than L/175. The third option allows for structural calculations which must follow the procedures stated in Engineering Design Rules set forth by the Fenestration and Glazing Industry Alliance (FGIA)/AAMA 2502-19. Those calculations include:

• Area moment of inertia, bending moments, and section modulus of the mullion element
• Deflection of the mullion element (not to exceed L/175 for spans of 13 feet, 6 inches or less and L/240 + ¼-inch for spans more than 13 feet, 6 inches at the given design pressure)
• Extreme fiber stress (structurally calculated)

AAMA 450-20 Update
In January 2021, FGIA announced an update to the mulled assemblies document. AAMA 450-20, renamed the standard as “Performance Rating Method for Mulled Combination Assemblies, Composite Units, and Other Mulled Fenestration Systems.” It describes test procedures and calculation procedures for determining the performance of mulled fenestration systems for both factory-assembled or field assembly with parts and instructions supplied by the fenestration product manufacturer. Changes and additions have been made to deflection limits, connection strength, load application, and the anchorage of mullion elements.

One particularly notable change is with composite units, which have been brought fully into the scope of the standard, together with storefront and curtainwall fenestration products and side-hinged doors. (A composite unit indicates two or more independent fenestration units, each of which must meet appropriate NAFS requirements, utilizing integral mullions within a single continuous outer main frame.)

With so many different available configurations of mulled fenestration systems, analysis and testing can be overwhelming. For that reason, a key component of AAMA 450 is the allowance of “product grouping” for the purpose of qualifying multiple designs with a single test or evaluation. While other published methods, such as AAMA 2502-19, Comparative Analysis Procedure for Window and Door Products, allow rating of untested sizes, AAMA 450-20 allows rating of untested configurations. The product group can include several different types of operating windows of different performance classes that are combined in a variety of ways using the same mullion profile. One test may qualify all possible combinations of the window types based on the weakest configuration.

Wind-load Calculations for Mulled Assemblies

We all understand that windows are one of the most vulnerable parts of a building when it comes to wind damage. When wind strikes a building, it creates a pressure differential on the building envelope that can cause windows to break or even be blown out of their frames, posing a safety hazard and potential for significant damage to the building and its contents.

Mulled window assemblies are particularly susceptible to wind damage because they create larger surface areas for the wind to act upon. The wind load on a mulled window assembly is greater than that on a single window unit, and the assembly may also have weaker points at the mullions where the units are joined together.

Understanding wind load for mulled window assemblies allows architects and engineers to design window systems that can withstand the forces of wind and protect the building and its occupants. By calculating the wind load on the windows and specifying windows that meet the necessary wind-load rating, architects can ensure that the windows are strong enough to resist the forces of wind and prevent damage or injury. Additionally, proper installation techniques and detailing, such as using appropriate framing and mullion design, can further increase the strength of the window system and reduce the risk of wind damage.

Mulled window assemblies may require additional structure in situations where the expected wind loads exceed the capacity of the window system to withstand them. This can occur in areas that are prone to high winds or in buildings with large window openings that are exposed to wind. One situation where additional structure may be required is when the wind loads exceed the design pressure rating of the windows. In this case, the window system may need to be reinforced with additional framing, such as steel or wood bracing, to increase its strength and rigidity.

How might an architect detect that a mulled window assembly needs additional structure? As a general rule, if the assembly creates a 3-way or 4-way intersection and it exceeds 6 feet in any one direction, additional structure is necessary. Other considerations for determining if additional structure is required would be looking at project site location in terms of terrain and region type. For instance, if the project is in a coastal region or Tornado Alley, it is clearly more susceptible to wind-load issues. And it is also more susceptible if it is on a hill or bluff, in an open field, or next to any body of water, not just oceanside.

Architects must also consider the type of building. High-rise buildings and commercial buildings with large storefronts or curtain walls tend to require additional structure. But as big-glass designs for residential homes become more prevalent, architects will need to be ever more aware of additional durability considerations for window assemblies.

Once it is determined that additional structure is necessary, architects should consult with structural engineers or other licensed design professionals to perform a wind-load calculation and analysis. Structural engineers use specialized software and engineering principles to calculate the expected wind loads on a building based on factors such as the building's location, height, and design. Architects will need to supply the project site address, building elevation drawings, and window assembly size.

The Project Site Address
The project site address will determine the location wind speeds, which will be entered into the calculator. Location wind speeds are based on the specific geographic location of the building or structure. This information is typically obtained from the ASCE 7 standard, which provides detailed information on the wind speed maps for the U.S. and other countries.

The ASCE 7 standard provides wind speed maps that divide the U.S. into zones based on the likelihood of encountering high winds. These maps are based on historical weather data and other factors, such as topography and surrounding terrain, that can affect wind patterns.

To determine the location wind speed for a particular building or structure, the design professional first determines the zone in which the building is located and then consults the wind speed maps or other sources that provide information on local wind conditions.

The project site address will also determine the building’s exposure category. The exposure category is based on the building's location and is used to determine the level of wind exposure that the building will experience.

Exposure categories are:

Category B

• Typical urban or suburban area
• Building and trees the same height or taller

Category C

• Open areas
• Buildings taller than their surroundings

Category D

• Waterfront with 1-mile exposure